1. Field of the Invention
The present invention is related to the field of electric wireline well logging. More specifically, the present invention is related to a means for determining the true depth of a well logging tool within a wellbore when the tool is extended into the wellbore by means of an electrical cable.
2. Description of the Related Art
Electric wireline logging cables are used to convey various types of measuring instruments into wellbores penetrating earth formations. The instruments generate signals which are related to physical properties of the earth formations. A record of the properties of the earth formations is made at a plurality of depths within the wellbore. The record is typically made while pulling the instrument out of the wellbore by reeling the logging cable onto a winch or similar spooling device, while simultaneously recording the signals generated by the instruments. The record of the measurements is thus made to correspond to the depths within the wellbore at which the measurements were made.
Measurement of the depth of the instrument in the wellbore is typically accomplished by using a calibrated wheel placed in frictional contact with the cable. The calibrated wheel turns correspondingly with the amount of linear motion of the cable past the wheel as the cable is moved into or out of the wellbore by the winch. The wheel can be rotationally coupled to a mechanical counter calibrated to indicate the length of cable moved past the wheel, or the wheel can be coupled to an encoder connected to a counter or computer for electronically indicating the length of cable moving past the wheel.
Calibrated wheels can accurately determine the total length of cable which has been moved past the wheel into the wellbore, but the true depth of the instrument in the wellbore may not correspond exactly to the total length of cable moving past the wheel because the cable is subject to stretch as tension on the cable varies.
The tension on the cable is affected by, among other things, the total weight of the cable disposed within the wellbore, which can be as much as 500 pounds for each 1000 feet of cable, the weight of the instrument when it is inserted into the wellbore, which can vary depending on how much of the instrument volume is enclosed air space and on the density of a fluid which may fill the wellbore, and friction caused by movement of the instrument against a wall of the wellbore.
Friction is the least predictable of the causes of tension on the cable as it is moved into and out of the wellbore because the wall surface of the wellbore has an unknown degree of roughness and the earth formations penetrated by the wellbore have unknown frictional coefficients. Fluid which typically fills the wellbore, called drilling mud, can have varying viscosity at different depths within a particular wellbore, making determination of friction even more difficult.
The measurements made by the instrument can have been made at depths as much as twenty feet or more different from the depth indicated by the calibrated wheel because of tension induced stretch in the cable as the instrument is pulled out of the wellbore.
A method for compensating the depth measurement for the amount of stretch in the cable is described for example in U.S. Pat. No. 4,803,479 to Graebner, et al. The method disclosed in the Graebner patent includes making a measurement of a shift in the phase of an electrical signal sent through the entire cable and returned to equipment at the earth's surface, the phase shift measurement related to the phase shift of the same electrical signal sent through a reference cable disposed at the earth's surface having invariant length. In the method of the Graebner patent, phase shift in a constant frequency electrical signal depends only on the change in transmission time of the signal, so phase shift corresponds to a change in the length of the electrical conductors in the cable.
A limitation of the method of the Graebner et al '479 patent is that the change in the length of the electrical conductors in the cable may not correspond exactly to a change in the length of the cable. Electrical logging cable typically comprises a plurality of insulated electrical conductors covered by helically-wound steel armor wires. A logging cable typically comprises seven conductors, six of the conductors being helically wound around the seventh conductor. When such a multiple conductor cable is stretched, some of the stretch is consumed by unwinding the helically wound conductors, so the cable length increases more than the length of the helically-wound conductors increases.
Another limitation of the method disclosed in the Graebner et al '479 patent is that the ratio of change in cable length to the phase shift of the electrical signal, called the scale factor, must be determined for each particular cable because electrical signal transmission properties can vary somewhat among different cables.
A still further limitation of the method disclosed in the Graebner et al '479 patent is the need to use of an additional conductive means at the earth's surface to provide a fixed length phase reference for comparison of phase change in the logging cable. A substantial length of cable to be used as a fixed length reference can occupy a significant storage space, which can be impractical.
It is also quite common to cut small lengths, such as 100 to 300 feet, from the end of a particular cable which is lowered into the wellbore as that end of the cable becomes worn or damaged. In other circumstances, the cable must be cut in order to retrieve an instrument which has become stuck in the wellbore, the cut cable later reassembled by splicing. Each time a cable is cut, the scale factor may have to be again determined by imparting a known amount of stretch to the cable and measuring the phase shift caused by the known stretch. It is difficult to recalibrate the scale factor at the wellbore location since equipment intended to impart a known stretch to the cable typically can be located only at a specialized facility.
A still further limitation of the system disclosed in the Graebner et al '479 patent is that the accuracy of the measurement of phase shift declines rapidly with increasing frequency of change in length of the cable. Higher frequency changes in the amount of cable stretch can be caused by "stick-slip" motion of the logging tool, as the combination of gravity and friction of the wellbore momentarily overcomes the upward pull of the logging cable, only to be violently released in a spring-like motion as the frictional force is overcome when the upward tension on the cable builds sufficiently.
Another method of determining the depth of logging tools in a wellbore is described in U.S. Pat. No. 3,490,149 issued to Bowers. The system disclosed in the Bowers '149 patent includes an accelerometer for measuring acceleration of the logging tools coaxial with the wellbore. Acceleration measurements of the logging tools coaxial with the wellbore are doubly integrated to provide a determination of change in axial position of the logging tools. The change in axial position determined from the doubly integrated accelerometer measurements is used to adjust the measured position of the tool as determined by measurements of the amount of cable which has passed a device for measuring the amount of cable extended into the wellbore. A drawback to the system disclosed in the Bowers '149 patent is that the doubly integrated acceleration measurements typically must be band limited by a filter to remove DC and very low frequency AC output from the accelerometer to correct for "drift in the zero reference" (also known in the art as bias error). The considerations in selecting the lower cutoff frequency are described in the Bowers '149 patent at col. 6, lines 25-71. If the acceleration on the tool falls below the cutoff frequency of the filter, then low frequency accelerations on the tool as may be caused by forces such as friction, which changes the tensile force on, and therefore the length of, the cable, may go undetected. The system disclosed in the Bowers '149 patent therefore is useful only to correct depth measurements for higher frequency accelerations on the logging tools.
An improvement on the method disclosed in the Bowers '149 patent is described in U.S. Pat. No. 4,545,242 issued to Chan. The system disclosed in the Chan '242 patent includes feedback amplifiers to decrease an error signal generated in the process of integrating accelerometer measurements to determine the true position of the logging tools in the wellbore. The system disclosed in the Chan '242 patent, however, still suffers the limitation of having substantially no system response below the lower cutoff frequency of a filter applied to the output of the accelerometers. The systems disclosed in Bowers '149 and Chan '242 are unable to provide accurate depth information in the event the electrical cable is "stretched" at frequencies below the cutoff of the filter applied to the accelerometer.
It therefore is an object of the present invention to provide a system for measuring the depth of a logging tool in a wellbore which measures the change in length of the logging cable, wherein the change in length of the logging cable can be calibrated for changes in the electrical properties of the cable.
It is a further object of the present invention to provide a system for determining the depth of a logging tool in a wellbore without the use of a fixed length reference cable at the earth's surface.
It is still a further object of the present invention to provide a system for determining the depth of logging tools in a wellbore which provides substantial response in a frequency range excluded by filters used in accelerometer-based systems, and in a frequency range excluded by the response of a phase detector in phase-shift detecting systems.